The soil down here is mostly this fine sand, almost the consistency of powdered sugar. Naturally the the trees to tend to put out quite long roots without many feeders around the trunk. I don't really have anything else available to me, so the soil that is filling my raised bed is predominantly the natural sandy soil and compost, with some cheap cat litter thrown in for moisture retention. As long as I keep the beds well watered and fertilized, will I have to worry too much about the plants sending out long root systems? Or will they keep a more condensed root ball?

The reason that plants in your native earth send out long roots is because the soil is relatively infertile. I think if you're using about 2/3 good compost and 1/3 sand, you'll see decent results. But if you can use something other than sand that keeps the soil open, I think it would be better.

A few years ago, I filled my raised beds with 3 or 4 different types of compost from a box store and about 25% sifted bonsai soil fines (especially pumice) and used bonsai soil. Naturally aggressive rooting trident maples (originally in 6" pots) had nice, dense 2' to 2.5' rootballs at the end of 2 years in this mix, and they put on tremendous growth.